Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore into the subterranean formation. Potentially, during the drilling process, a string of pipe (e.g., casing) is run in the wellbore and cemented in place. Cementing is typically performed whereby a cement slurry is placed in the annulus outside the casing and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus.
In the performance of such a cementing operation, or in the performance of one or more other wellbore operations (e.g., a drilling operation, a stimulation operation, a completion operation, a fluid-loss control operation, production, or combinations thereof), it may be desirable to obtain data from within the wellbore, for example, data related to the conditions within the wellbore or data related to the operation or performance of downhole tools positioned within the wellbore.
Such data may include geology, rate of rock penetration, inclination, azimuth, fluid composition, temperature, and pressure, among others. Special downhole assemblies have been developed to monitor subsurface conditions. These assemblies are generally referred to as Logging While Drilling (LWD) or Measurement While Drilling (MWD) assemblies. LWD and MWD assemblies can be carried by downhole tools or any other apparatus that is placed downhole, and are able to store or transmit information about subsurface conditions for review by drilling or production operators at the surface.
A variety of technologies have been proposed or developed for downhole communications using LWD or MWD. In a basic form, MWD and LWD assemblies can store information in a processor having memory. The processor can be retrieved, and the information downloaded, later, when the downhole tool is removed from the wellbore.
Several real time data telemetry systems have also been proposed. Some involve the use of physical cable such as a fiber optic cable that is secured to the casing string. The cable may be secured to either the inner or outer diameter of the casing string. The cable provides a hard wire connection that allows for real time transmission of data and the immediate evaluation of subsurface conditions. Further, these cables allow for high data transmission rates and the delivery of electrical power directly to downhole sensors. As an alternative to such a wired system, nodes have been placed along a casing string to utilize near-field communications (NFC), to communicate one or more signals between nodes and up the casing string to the surface. The node-to-node communication allows transmission of data up the wellbore. The use of radio frequency signals has also been suggested.
These systems all require data to be transmitted over a long distance through multiple nodes. The data signal that reaches the surface is only as good as the signal that can be passed between nodes. Thus, a need exists for a data transmission system that can transmit data between communication nodes.